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TECHNICAL INFORMATION ON CHELATE STABILITY
The stability (tendency to dissociate in
solution) of metal complexes depend on both the metal ion
and the ligand:
- Metal ion - increases stability
of the complex with:
•
Increasing ion charge - metals have
different charges depending on their oxidation state E.g. Mg2+, Fe(II) 2+
(Ferrous), Fe(III) 3+ (Ferric), Zn2+, Cu(I)+ (Cuprous), Cu(II)2+ (Cupric)
•
Decreasing size
•
Increasing electron affinity
- Ligand - factors increasing the stability of
the complex:
• Basicity of the
ligand - coordination compounds are formed as a result of acid-base reactions,
where the metal ion is the acid and the ligand is the base
• Number of metal-chelate rings per ligand
• Size of the
chelate ring
• Steric effects
• Resonance effects
Note that the percentage of a metal formed /present as a particular complex in
solution depends on:
- Its environmental pH
- The solubility
constant of the complex - which is affected by temperature
Stability Constants of Metal
Complexes
A measure of the strength of the reaction between the specific metal
with the specific ligand in solution. Values range from 0-20 - the lower
the number, the weaker the strength of the reaction, and the more likely
the metal will separate / dissociate from its ligand in solution.
Negative Values - 1: metal-ligand is
soluble in water and will readily dissociate yielding all of the metal in ionic
form in pH levels of 2 (stomach acid) to 7.4 (extracellular body fluids: serum,
lymph)
Values of 1-3: Metal-ligands are soluble and ~5%
ionic at pH 7.4
Values 3- ~6: metal-ligand will likely dissociate
in stomach acid, but not in physiological pH 7.4
Values >6: less and less metal-ligand dissociates
at any pH and such compounds are unusable by the body
Dissociation ability is inversely affected by pH and
temperature
- the stomach acid has a pH of 2-3, duodenum (pH 6), increasing to pH 7.4 in the
terminal ileum. The pH drops to 5.7 in the caecum, but again gradually
increases, reaching pH 6.7 in the rectum.
Ref
Amino Acids
Amino acids have three basic parts: the amino ‘group’
(i.e.,
group of atoms), the acid group, and the R-group
- It is the R-group that
determines the name and specific character of an amino acid -determining, for
example, whether the amino acid is aspartic acid or lysine or tryptophan.
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